The invention lies in the field of optical waveguides and can be applied to the configuration of connections between optoelectronic components, in particular, light-emitting components such as laser diodes and multimode optical waveguides.
During the direct optical coupling of light-emitting components that have weak wave-guidance, and, thus, little expansion of the pencil of rays that occurs, to multimode optical waveguides, illumination of the core of the respective optical waveguide is only partially achieved. By way of example, this is the case during the coupling of a laser diode of the VCSEL type and a multimode optical waveguide. During the direct coupling of such a laser diode without an imaging system to a multimode waveguide with a graded-index profile, as is predominantly used in transmission in the field of data communication, only the low-order modes are excited in the optical waveguide. When the light beam emerges from the optical waveguide, this leads to a light beam with little angular expansion so that the guidelines for so-called laser safetyxe2x80x94which is intended to serve to avoid damage to eyesxe2x80x94can be exceeded even at relatively low optical radiation levels. Such a problem becomes particularly acutely apparent in the case of modules in which a plurality of light-emitting components are disposed as an array because, in this case, a plurality of optical waveguides are closely adjacent and, accordingly, the radiated light cones of the individual waveguides are superposed from one another such that extremely high radiation levels can arise in the detection of the emitted radiation for evaluation of laser safety.
For coupling light-emitting or light-receiving components to an optical waveguide, there exists in the prior art a placement of a stub of an optical fiber between the end of the optical waveguide and the component and an inclination of that end side of the fiber stub that is directed toward the component relative to the geometrical axis of the fiber stub to preclude reflections. See U.S. Pat. No. 5,937,122 to Ohki et al. For coupling light into a multimode waveguide, use has also been made of a fiber stub made from a monomode optical waveguide, that end face of the fiber stub that is directed toward the optical waveguide bearing in a planar manner on the end face of the optical waveguide. See German Published, Non-Prosecuted Patent Application DE 196 45 295 A1, corresponding to U.S. Pat. No. 6,044,188 to Kropp.
It is accordingly an object of the invention to provide an optical coupling system for coupling a radiation source to a multimode optical waveguide that overcomes the hereinafter-mentioned disadvantages of the heretofore-known devices of this general type and that configures the coupling system such that the highest possible optical power can be transmitted whilst ensuring so-called laser safety.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an optical coupling system for coupling a radiation source emitting radiation as a light beam along an optical axis in an emission direction to a multimode optical waveguide having acceptance angle, the optical coupling system including an optical fiber having a stub, an optical core, an optical cladding, an end-side coupling-out face, an end-side coupling-in face, and a geometrical axis, the stub being a multimode step-index waveguide having an acceptance angle greater than a half-value width of an angular distribution of the radiation emerging from the radiation source, the stub to be disposed downstream of the radiation source with respect to the emission direction, the coupling-in face to be directed toward the radiation source, the coupling-out face to be directed toward and directly connected to the optical waveguide, and the coupling-in face to be associated with the radiation source to couple the light beam into the optical fiber and to incline the light beam, as a coupled-in light beam, relative to the geometrical axis by an-angle greater than 2xc2x0 and less than the acceptance angle of the optical waveguide.
The invention provides that the radiation source and the coupling-in face of a multimode fiber stub are associated with one another such that a light beam emitted along the optical axis of the radiation source as coupled-in light beam is inclined relative to the geometrical axis of the optical core of the fiber stub by an angle that is greater than 2xc2x0 and less than the acceptance angle of the optical waveguide.
The inclinationxe2x80x94provided according to the inventionxe2x80x94of the main direction of the light beam in the fiber stub and the use of a fiber stub made of a multimode step-index waveguide have the result that the radiation, on account of the light guidance in the step-index fiber, even on a very short length of about 5 to 10 mm, is homogeneously distributed both over the cross-sectional area and in the angular distribution such that a radiation with an expanded beam angle emerges at the exit face of the pin stub. In such a case, the luminous power is also distributed over a large angular range so that the intensity measured in a constricted angular range according to the guidelines for laser safety becomes particularly low. This is particularly advantageous in the case of multiple systems to ensure so-called laser safety even when the individual optical waveguides have a spacing of about 250 xcexcm.
The extent of the inclinationxe2x80x94provided according to the inventionxe2x80x94of the main direction of the coupled-in light beam in the fiber stub substantially depends on the half-value width of the angular distribution of the radiation emerging from the radiation source, and of the acceptance angle of the fiber stub and also on the acceptance angle of the optical waveguide to be connected. With account additionally being taken of the coupling efficiency between the radiation source and the optical waveguide, the extent lies in the range of about 2 to 10xc2x0.
The coupling-in of the light beams into the fiber stub, the coupling-in being provided according to the invention and deviating from the course of the geometrical axis of the optical core of the fiber stub, that is to say, being slanted, can be realized in different ways.
The simplest possibility in terms of production engineering lies in correspondingly inclining the optical axis of the radiation source relative to the geometrical axis of the optical core of the fiber stub.
With regard to the configuration of the components on a carrier substrate, it may also be advantageous, however, to dispose the optical axis of the radiation source and the geometrical axis of the optical core of the fiber stub coaxially with respect to one another and to incline the coupling-in face of the fiber stub relative to the geometrical axis thereof.
What is particularly expedientxe2x80x94due to its axially short structural lengthxe2x80x94is the variant of inclining the coupling-in face of the fiber stub relative to the geometrical axis of the optical core of the fiber stub and configuring the radiation source radially with respect to the fiber stub such that the radiation is reflected at the coupling-in face. Such a coupling-in is disclosed, in principle, in European Patent Application EP 0 404 053 B1, corresponding to U.S. Pat. No. 5,023,447to Masuko et al., but not in conjunction with slanted coupling-in.
To be able to couple as much luminous power as possible into the optical fiber adjoining the fiber stub, in accordance with a concomitant feature of the invention, the diameter of the optical core of the fiber stub should be at most 1.1 times the diameter of the optical core of the optical waveguide.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an optical coupling system, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.